While hydrogen has wide potential as a fuel, a major drawback in its utilization, especially in mobile uses such as the powering of vehicles, has been the lack of acceptable lightweight hydrogen storage medium. Certain materials and alloys in solid state have the ability to absorb and desorb hydrogen. These materials have been considered as a possible form of hydrogen-storage, due to their large hydrogen-storage capacity. Storage of hydrogen as a solid hydride can provide a greater volumetric storage density than storage as a compressed gas or a liquid in cryogenic tanks. Also, hydrogen storage in a solid hydride presents fewer safety problems than those caused by hydrogen stored in containers as a gas or a liquid. Solid-phase storage of hydrogen in a metal or alloy system works by absorbing hydrogen through the formation of a metal hydride under a specific temperature/pressure or electrochemical conditions, and releasing hydrogen by changing these conditions. Metal hydride systems have the advantage of high-density hydrogen-storage for long periods of time.
A desirable hydrogen storage material must have a high storage capacity relative to the weight of the material, a suitable desorption temperature/pressure, good absorption/desorption kinetics, and good reversibility.
A low hydrogen desorption temperature is desirable to reduce the amount of energy required to release the hydrogen from the material, as spending a great deal of energy to desorb hydrogen reduces the efficiency of the system. Furthermore, materials having relatively low hydrogen desorption temperature are necessary for efficient utilization of the available exhaust heat from vehicles, machinery, fuel cells, or other similar equipment.
Good reversibility is needed to enable the hydrogen storage material to be capable of repeated absorption-desorption cycles without significant loss of its hydrogen storage capabilities. Good absorption/desorption kinetics are necessary to enable hydrogen to be absorbed or desorbed in a relatively short period of time. For example, magnesium hydride has relatively high hydrogen storage capacity but the desorption kinetics of the magnesium hydride is less than desirable at room temperature. Even at higher temperatures it is difficult to desorb all of the hydrogen stored in hydride form. Therefore, it is necessary to find a material or family of materials that will store more hydrogen with good reversibility and improved absorption/desorption kinetics.
A family of complex aluminum hydrides such as NaAlH4, LiAlH4, and Mg(AlH4)2 have good theoretical reversible capacities. This family of complex aluminum hydrides are generally referred to as alanates. In practice, the reversibility of the alanates could not be achieved until recently when it was found that the addition of a small amount of a titanium catalyst made them reversible under certain conditions. However, doping with some titanium catalysts reduces the hydrogen storage capacities due to the presence of metal salt byproducts. Accordingly there is a desire for a hydrogen storage material with the advantageous features of the known alanate/titanium catalyst system but with greater hydrogen storage capacity. Additionally there is an ongoing need to reduce the temperature at which the absorption-desorption cycle takes place.